In the digital television signals for broadcasting high-definition television (HDTV), each data field contains 314 data lines, and the fields are consecutively numbered modulo-two in order of their occurrence. Each line of data starts with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The lines of data are each of 77.7microsecond duration, and there are 832 symbols per data line for a symbol rate of about 10 megabits/second.
The initial line of each data field is a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-random sequence (or "PR-sequence") followed by three 63-sample PR sequences. This training signal is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. The occurrence of the PR sequences can be used to establish beginning of data field indications that can be used for synchronizing the counting of the lines of data, to aid in the identification of the content of each data line.
In order to count the data lines in each successive non-overlapping pair of data fields, it is desirable to determine with a high degree of confidence the occurrence the line synchronization code group of four symbols having successive values of +S, -S, -S and +S at the beginning of each data line. A matched filter can be used to detect the occurrence of the line synchronization code group, but undesirably will also respond to other successive values of +S, -S, -S and +S randomly occurring in the digital television data. Also, if noise interferes with the four symbols in the line synchronization code group at the beginning of each data line, the matched filter response may fail to be generated.
A matched filter for detecting detect the occurrence of the line synchronization code group is constructed as follows. A tapped delay line has an input tap to which the digital television data are supplied, an output tap, first and second intermediate taps, a first symbol latch having an input connection from the input tap and having an output connection to the first intermediate tap, a second symbol latch having an input connection from the first intermediate tap and having an output connection to the second intermediate tap, and a third symbol latch having an input connection from the second intermediate tap and having an output connection to the output tap. The signals at the input tap, the first and second intermediate taps and the output tap are combined in 1:(-1):(-1):1 ratio to generate a combined response. A window detector receives the combined response and generates a response to the combined response whenever successive values of +S, -S, -S and +S occur in the digital television data supplied to the input tap of the first tapped delay line. If the combined response is generated by signed addition of the responses, so as nominally to result in a 4S response to the line synchronization code group, the window detector responds to a window one code level wide centered on 4S response.
The invention resorts to correlation filtering to obtain reinforced response to line synchronization code groups as compared to other successive values of +S, -S, -S and +S randomly occurring in the digital television data. The correlation filtering also provides for filling in occasionally missing responses to line synchronization code groups.